Photosensitive elements for image-reproduction are well-known in the graphics arts industry. Such elements are usually exposed to radiation through an image-bearing transparency, such as a color separation transparency, to produce an image that is either a positive or negative of the transparency.
Such photosensitive elements are widely used in off-press color proofing to simulate the images produced by printing. In a surprint proof, for example, all of the colored images are superposed, by multiple exposure, lamination, or transfer, on a single support. The colored images cannot be separated and viewed individually.
Many of the commonly used photosensitive elements used in image reproduction require development by organic solvents or an aqueous base. Use of organic solvents is frequently undesirable because of flammability, toxicity, and waste disposal considerations. The use of aqueous base is frequently undesirable because of toxicity, waste disposal, and corrosion problems. Thus there continues to be a need for improved photosensitive elements capable of reproducing images on a variety of receptors and that can be washed-out using aqueous solutions, preferably ordinary tap water.
Various processes are known for producing copies of images involving photopolymerization and thermal transfer techniques, such as those disclosed in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; 3,574,049 and 3,607,264. In these processes, a photopolymerizable layer coated on a support is imagewise exposed to a photographic transparency. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element, and at least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the unexposed image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a toner. All of these processes necessitate the use of specially treated final receptor sheets and are not applicable for obtaining a color proofing image on a paper stock.
In the production of color printing proofs, transfer the color proofing image to actual printing paper stock is highly desired since the resulting proof usually will afford a more faithful and accurate reproduction of the proofing image to that produced on the press. Since paper stock is not resistant to the chemical processing steps required for obtaining an image, however, the image must first be formed and then transferred to the paper stock to create the color proof.
Methods for forming a color image on certain paper stocks are described in U.S. Pat. Nos. 4,766,053, 5,109,471 and 5,059,509. These methods involve transferring the image to an image-receiving sheet and then further transferring the image to a permanent support. For multicolor images, these methods are disadvantageous in that the individual color images must be laminated together in register after the individual images are formed. The lamination step requires a great deal of skill and care for high quality reproductions, and becomes increasingly difficult as the size of the work increases. Furthermore, the developer for these processes is a basic solution optionally containing organic components that can present waste disposal, corrosion, and toxicity problems.
Another method for forming a color image on diverse paper stocks is described in U.S. Pat. No. 5,094,931. In this method, a photosensitive element on a substrate is laminated to a temporary element and exposed, with substrate then being removed before development. After development, the steps are repeated with another color. A protective element, including an additional adhesive layer, is then laminated to the colored image. After removal of the support of the temporary element, the colored image with protective element is laminated to a permanent receiver, which can be a diverse paper stock. A disadvantage of this method is that it includes at least one extra layer in the color image composite that is nonessential to the color image and which detracts from the quality of the image reproduction. A second disadvantage is that the developer is a basic solution that can present waste disposal, corrosion, and toxicity problems.
It is an object of the present invention to provide a process to obtain a color image on virtually any paper stock in which the image contains the minimal number of layers essential for image formation, and for which the registration of different colored images is accomplished easily. Such a color image will provide a color proof that is a more faithful reproduction to an on-press print. It is also desired to do the development steps using tap water as the liquid developer, to minimize or eliminate the waste disposal, corrosion, and toxicity problems associated with the imaging process.